Table 1.
Subject data.
Figure 1.
Locomotor-ventilatory interactions.
(A–B) Typical head acceleration (top) and ventilatory flow (bottom: expiration positive, inspiration negative) during quiet standing (A), and moderate speed treadmill running (B). Note the high frequency oscillations in ventilatory flow during running. (C) Schematic illustration of the ‘visceral-piston’ model for human locomotor-ventilatory interactions. Red arrows indicate muscle actions during inspiration and expiration.
Figure 2.
Step-driven ventilatory flows and volumes.
(A) Grand mean of step-driven ventilatory flow across subjects (mean ± 95% confidence interval for 12 subjects, see also individual examples in Fig. 5). Expiration is positive. (B) Mean ± SD of step-driven volume as a percentage of total concurrent ventilatory volume (Vtot), during level moderate speed running (N = 12, black dots show data from individuals). Step-driven flow and volume data included here are from subjects with variation in the phase locking between steps and breaths (Table 1).
Figure 3.
Step driven flow depends on ventilatory phase.
(A) Variation in the magnitude of step-driven flows is apparent, particularly during low frequency breathing, as shown here during 3∶1 (strides per breath) coupling. Note the reversal in flow in late expiration (asterisk). Dashed vertical lines indicate footstrike events (data from subject 3). (B) Average step-driven flow (means ±95%CI) for a representative subjective at four points in the ventilatory cycle: 1) early expiration, high lung volume (90% Vmax), 2) late expiration, low lung volume (10% Vmax), 3) early inspiration, low lung volume (10% Vmax) and 4) late inspiration, high lung volume (90% Vmax).
Table 2.
Step-driven ventilatory volume, as a fraction of concurrent volume, during inspiration and expiration.
Figure 4.
The net effect of step-driven ventilation shifts from synergistic to antagonistic in each ventilatory half-cycle.
We show net step-driven volumes (in milliliters per step) as a function of the tidal volume at the time of footstrike during inspiration (left) and expiration (right), averaged across individuals (mean ± 95% CI, N = 12). At low lung volumes, the inspiratory pulse at footstrike is larger, leading to a net inspiratory effect. At high lung volumes, a larger expiratory pulse occurs, leading to a net expiratory effect.
Figure 5.
Subjects prefer to initiate ventilatory transitions at phases that assist rather than imped flow.
The distribution of ventilatory transitions relative to the step cycle is non-uniform (Table 3). Here we show the net bias in ventilatory transitions relative to step cycle (left axis and bars), with transitions to expiration as positive, transition to inspiration as negative. The ‘net bias’ value is the number of expiratory transitions minus the number of inspiratory transitions in each phase bin. Step-driven flow is overlaid for reference (right axis and lines). Data from 3 individuals illustrates typical variation between strongly coupled (A), variably coupled (B) and uncoupled (C) subjects. Although variation exists, the timing of transitions is clearly non-random, and exhibits some correspondence to the step-driven flow.
Table 3.
Mean phase and dispersion (in degrees) of ventilatory transitions relative to the step cycle.
Figure 6.
Phasing of steps relative to breaths has a significant effect on the duration of ventilatory transitions.
(A) Examples of ventilatory transitions timed with preferred and assistive phases of the step cycle, facilitating rapid transitions, and (B) timed with an avoided and antagonistic phases of the step cycle. Dashed vertical lines indicate zero-crossings of ventilatory transitions. The transition durations (T50) are calculated between the black dots indicating the times of 50% peak flows. (C) Transition times averaged across individuals (mean ± s.e.m), comparing breaths at ‘avoided’ and ‘preferred’ phase relationships. Preferred refers to the most used phase bin (e.g., see Fig. 5), and ‘avoided’ refers to the least used phase bin that was represented in the data.